1. Field of the Invention
The present invention concerns a method to separate a signal component of a cerebrospinal fluid from other signal components in the acquisition of MR images of an examination subject. The invention is particularly suitable for use in the acquisition of MR images of the brain.
2. Description of the Prior Art
Cerebrospinal fluid (CSF) is a clear, colorless liquid that fills the cerebral ventricles and the spinal canal and protects the brain and the spinal cord from shocks, and serves as a medium for material exchange. Cerebrospinal fluid has a long T2 relaxation time which leads to a very bright signal in T2 weighted images of the brain. Pathologies in the brain, for example multiple sclerosis (MS), likewise have a relatively long T2 relaxation time, such that the strong signal of the cerebrospinal fluid can occlude the signals of the pathological tissue in T2-weighted images of the brain. For this reason the signal portion of cerebrospinal fluid should be suppressed for a precise diagnosis.
In order to eliminate the CSF signal portions, it is known to use an IR (inversion recovery) preparation pulse in connection with a long inversion time TI due to the long T1 relaxation time of the cerebrospinal fluid. However, this long IR preparation phase increases the total acquisition time of the images to a significant degree and leads to a reduced contrast between white and grey brain matter. For this reason the application of an IR preparation for T2-weighted images with CSF suppression is not possible.
Furthermore, M. Essig et al. “Assessment of Cerebral Gliomas by a New Dark Fluid Sequence HIRE (High Intensity Reduction)” in Proc. Intl. Soc. Mag. Reson. Med. 8, 2000, Page 386 discloses acquiring two images with different contrast in a single measurement, wherein one image is T2-weighted while the other image is very strongly T2-weighted. Almost exclusively CSF signal contributions exist in the very strongly T2-weighted image due to the long T2 time. In this method, the second image is used as a scale for the cerebrospinal fluid, wherein a CSF-suppressed T2-weighted image is generated by a subtraction of the two images. However, since pathologies as well as multiple sclerosis have a relatively long T2 time, the pathological signals can likewise be reduced by the subtraction of the two images.